The objectives of the proposed research project are directed toward gaining a better understanding of the biology of teratocarcinomas. We will use newly developed inbred strains of mice with high susceptibilities to teratocarcinogenesis in the testis and ovary to study genetic and environmental factors involved in the neoplastic potential of male primordial germ cells, oocytes, and undetermined embryonal cells. We will obtain evidence that may demonstrate that embyro-derived teratocarcinomas are derived directly from embryonic cells rather than from cells that have become determined to form primordial germ cells. We will perform experiments that will support the concept that precursors of germ cells have not yet been determined in the ectoderm of 6-day embyros. This extremely important question has not yet been amenable to attack.